Probably nothing would happen. There would be some low toned audible gasp followed by an awkward silence. The meeting would quickly continue. Should you have the balls to sit quietly in your seat till the conclusion of the meeting you'd very likely be staring at 2-3 elders soon as that closing prayer finished.
At an assembly it would cause a more notable audible disruption. Might even hear a few isolated but loud boo's from the less spiritual but enthusiastic and slightly mental types. If the conductor is particularly charismatic he might make a clever remark about how eager that person is to get into Satan's world. The friends would laugh then start clapping.
Would it be interesting. Yes !!
Though personally I'm not a fan of Hall crashing or any sort of disruptive display of disagreement. In my opinion its disrespectful.